1. Field of the Invention
This invention relates to an underwater towline and more particularly to a low-drag and high-strength towline for towing an object under water at high speeds.
2. Brief Description of the Prior Art
High drag forces on towed devices tend to cause the device to stream out near the water surface behind the towing vessel unless depressing forces are applied to submerge the device. These depressing forces are usually applied by means of flaps and control surfaces on the towed devices, wherein the resultant depressing forces and drag forces on the towed devices are carried as tension forces in the towing cables or faired towlines that extend between the ship and the towed devices.
Drag forces, which are also produced on the towlines, are generally a function of the towing speed, the size of the towline, and the shape of the towline. Attempts to reduce the resultant drag forces on the towlines include the fabrication of streamlined integrated towlines having rounded leading edge portions and tapered trailing edge portions, as exemplified, for example, by U.S. Pat. Nos. 3,304,364; 3,352,274; 3,443,020; 3,611,976; and 3,613,627. In the type of towlines disclosed by these natents, the load bearing member typically consists of parallel glass fibers embedded in epoxy matrix for strength purposes, and a fairing is constructed of material having a low modulus of elasticity. The load bearing and fairing members of the towlines are provided with a rubber impregnated cloth covering which serves to maintain the structural integrity and shape stability of the towline.
However, attempts to tow submerged devices at preselected depths and at predetermined orientation beneath the towing vessel have often been unsuccessful due to "kiting" instabilities and erratic deflections of the towline. For example, towing tests with towlines of the type shown in U.S. Pat. No. 3,613,627 have shown the towline to be susceptible to hydrodynamic and mechanical instabilities as well as shape asymmetries that produce excessive towline kiting. Further the highly streamlined glass fiber-epoxy tensile members which form the load-bearing member of the towline are structurally unstable when curved in the plane of the chordline, (i.e., the towline is bent in the direction of relative flow as must occur for equilibrium). This structural instability, which is compounded by the structural instability of the trailing rubber fairing portion, must be compensated for by the natural hydrodynamic stability forces produced on the towline surfaces by the water.
In prior towline analysis, the stabilizing forces have been treated as if the faired section were longitudinally rigid in the chordwise direction. However, in actuality, the elasticity of the fairing of the towline, which must be of a soft material to minimize the buckling instability occasioned by the forward curvature of the towline, leads to a substantial reduction in the hydrodynamic stabilizing moments and forces on the towlines. Prior analysis considered the shift of the center of tension in the fairing as the controlling factor. Also, small separations of the interface bond between the load bearing and fairing members of the towline have been known to produce irregular lateral displacements of the tapered trailing portion relative to the load-bearing member to produce a longitudinal shape asymmetry in the towline. This factor was also ignored in prior art analyses. However, shape asymmetry occurring in a portion of the fairing causes unbalanced hydrodynamic forces thereabout which cause the length of towline to deviate severely from the intended planar configuration and thus results in substantial loss of depth and control capability. Prior analyses also disregard the effect of the torques induced in the load-bearing member when displaced out of plane.